The present invention relates to a method for growing a nitride compound semiconductor. More particularly, it relates to a new and improved method for growing a GaN or other nitride III-V compound semiconductor.
GaN, AlGaN, GaInN and other nitride III-V compound semiconductors have band gaps ranging from 1.8 eV to 6.2 eV, and they theoretically may be used to provide light emitting devices capable of emitting infrared to ultraviolet light and for this reason, these metal nitride compound semiconductors are being studied by engineers.
To fabricate light emitting diodes (LED) or semiconductor lasers using nitride III-V compound semiconductors, it is necessary to stack layers of AlGaN, GaInN, GaN, etc. and to sandwich a light emitting layer (active layer) between an n-type cladding layer and a p-type cladding layer.
To grow a p-type GaN layer, for example, by metal organic chemical vapor deposition or other vapor phase growth in a conventional technique, trimethylgallium (TMG, Ga(CH3)3) as Ga source, ammonia (NH3) as N source, and cyclopentadienyl magnesium (CP2Mg) as a p-type dopant, for example, are supplied onto a heated substrate, such as sapphire substrate, SiC substrate or GaAs substrate, in hydrogen (H2) carrier gas or mixed gas containing H2 and nitrogen (N2), to grow a Mg-doped GaN layer by heat decomposition reaction. Since the Mg-doped GaN layer has a high resistance immediately after the growth, it is subsequently annealed in a vacuum or in an inert or inactive gas to change it into a p-type semiconductor layer. It is considered that the change into a p-type occurs because Mg in GaN is activated and releases carriers.
However, the carrier concentration of the p-type GaN layer obtained in the above-mentioned process is around only 3xc3x971017 cmxe2x88x923, and the resistance still remains undesirably high. Therefore, the use of a nitride III-V compound semiconductor in a semiconductor laser presents some difficulties. A first problem attendant on the use of a p-type GaN layer as a contact layer for the p-side electrode arises because the p-type GaN layer has a high resistance and a large voltage loss may occur in the p-type GaN layer when the laser is operated with a high electric current. A second problem arises because of a low carrier concentration of the p-type GaN layer, which causes a contact resistance as high as 10xe2x88x922 cm2 between the p-type GaN layer and the p-side electrode. This causes a voltage loss of approximately 10 V along the interface between the p-type GaN layer and the p-side electrode while the semiconductor laser is operated in a typical inrush current density, 1 kA/cm2, and causes a deterioration in laser characteristics. A third problem is that the need for an annealing step for activating the impurity after growth of the Mg-doped GaN layer causes an increase in the number of steps required to perform the manufacturing process.
The above problems concerning p-type GaN also apply to fabrication of a p-type layer of any nitride III-V compound semiconductor or, more generally, a nitride compound semiconductor, other than GaN.
It is therefore an object of the invention to provide a method for growing a nitride compound semiconductor, which provides fabrication of a p-type nitride compound semiconductor with a high carrier concentration, which does not need to be annealed after growth of the semiconductor to activate the p-type impurity therein.
After considerable research directed to solving these conventional technical problems, it has now been discovered that a p-type nitride compound semiconductor with a high carrier concentration may be obtained by preventing generation of active hydrogen during formation of the semiconductor and that it, is very important to select an appropriate nitrogen source material and an appropriate carrier gas.
In accordance with an embodiment of the invention, a carrier gas other than H2 is used preferably because H2, when caught into grown crystal, inactivates Mg used, for example, as a p-type impurity. Suitable carrier gases which may be employed in accordance with this embodiment are inert or non-reactive carrier gases, such as, argon (Ar), helium (He) and Nitrogen (N2), for example. From the economical viewpoint, N2 is most inexpensive.
In accordance with an embodiment, a nitrogen source should be used that does not release H2. This does not mean that the nitrogen source cannot contain a hydrogen group (xe2x80x94H). When both a methyl group (xe2x80x94CH3) and a hydrogen group (xe2x80x94H) are contained in a single molecule, there is a high probability that they join and form stable methane (CH4). Therefore, an essential condition of molecules to be used as a nitrogen source material is that the number of hydrogen groups should be equal to or less than the number of methyl groups, per molecule of the nitrogen source. For example, trimethylhydrazine, ((CH3)2 Nxe2x80x94NH(CH3)) is decomposed as 
and does not release hydrogen.
A nitrogen source material having ethyl (xe2x80x94C2 H5) groups instead of methyl groups behaves somewhat differently. For example, a possible decomposition reaction of diethyl amine (HN(C2H5)2) is
HN(C2H5)2xe2x86x92NH3+2C2H4xe2x80x83xe2x80x83(2)
Another possible decomposition reaction is: 
These are decomposition reactions that release hydrogen. In Equations (2) and (3), ethylene is produced. The decomposition and formation of ethyl groups into ethylene is called xcex2-elimination decomposition which is very liable to occur in organic metal compounds such as triethylgallium (TEG, Ga(C2H5)3). However, it is believed that xcex2-elimination decomposition is unlikely to occur in ethyl compounds containing group V elements such as As and N (for example, Appl. Organometal Chem. vol. 5, 331(1991)). For these materials, the decomposition reaction proceeds as follows:
HN(C2H5)2xe2x86x92N+C2H6+C2H5xe2x80x83xe2x80x83(4)
and the reaction does not release hydrogen. Therefore, ethyl groups may be regarded the same as methyl groups, and also other alkyl groups may be regarded the same as methyl groups.
Aromatic ring hydrocarbons having phenyl groups (C6H5xe2x80x94) are hydrocarbons which do not exhibit xcex2-elimination. Aromatic ring hydrocarbons are very stable, and need a large energy for their decomposition. Amines having a phenyl group coupled to nitrogen, e.g. phenylmethylamine (C6H5xe2x80x94NH(CH3)), is decomposed in accordance with the reaction formula: 
and does not release hydrogen. In this respect, phenyl groups may be regarded equivalent to methyl groups. The same also applies to higher order aromatic ring hydrocarbons, such as naphthalene. In general, however, they are disadvantageous because of a high vapor pressure.
Taking the above factors into account, nitrogen source materials suitable for obtaining p-type III-V compound semiconductors with a higher carrier concentration are NR3, NHR2, etc. as amine compounds, RNxe2x95x90NR, HNxe2x95x90NR, etc. as azo compounds, R.sub.2 Nxe2x80x94NH2, R2Nxe2x80x94NHR, R.sub.2 Nxe2x80x94NR2, RHNxe2x80x94NRH, RHNxe2x80x94NRxe2x80x2H, RHNxe2x80x94NRxe2x80x22 etc. as hydrazine compounds, and Rxe2x80x94N3, etc. as azide compounds, where R is an alkyl group or a phenyl group (xe2x80x94C6H5), and H is a hydrogen group (xe2x80x94H).
In accordance with the present invention, in an embodiment, the group III element materials used for the growth a nitride III-V compound semiconductor also do not release hydrogen. Trimethylgallium (TMG, Ga(CH3)3), for example, does not release hydrogen during decomposition, and therefore is suitable as a Group III elemental source.
However, triethylgallium (TEG, Ga(C2H5)3) is decomposed by a xcex2-elimination reaction, as follows:
Ga(C2H5)3xe2x86x92GaH3+3C2H4xe2x80x83xe2x80x83(6)
and generates galane (GaH3), leaving hydrogen on the grown surface. In this respect, TEG is considered an unfavorable material. However, considering that the amount of a group V material supplied, in general, is hundreds of times the amount of a group III material, if trimethylamine is used as a nitrogen material, methyl groups are theoretically hundreds times the amount of GaH3 and consume GaH3. That is, since the amount of supply of the group III material is overwhelmingly small, generation of hydrogen from the group III material can be disregarded.
Gallium chloride (GaCl) may also be used in a chloride method. From the review made above, there is substantially no restriction on group III materials.
In accordance with the present invention, in an embodiment, a new and improved method for growing a nitride compound semiconductor, comprising the step of:
growing a nitride compound semiconductor in vapor phase by using a nitrogen source material which does not release hydrogen during release of nitrogen.
In a preferred embodiment of the invention, the nitride compound semiconductor is grown in vapor phase in an inactive gas. Usable as the inactive gas are, for example, Ar, He and N2.
Typically used as the nitrogen source material is a nitrogen compound that contains a hydrogen group, an alkyl group and/or a phenyl group, and the number of the hydrogen groups is not more than the total number of the alkyl group or groups and the phenyl group or groups taken together. Preferred nitrogen compounds are selected from amine compounds, azo compounds, hydrazine compounds, azide compounds, and the like, and a suitable compound is selected from any of these aforementioned compounds, depending on the type of the nitride compound semiconductor to be grown. Especially preferred amine compounds are trimethylamine, dimethylamine, triethylamine and diethylamine.
In accordance with the invention, a nitride compound semiconductor is provided in the form of a nitride III-V compound semiconductor containing N and at least one group III element selected from the group consisting of Al, Ga and In. Typical examples of the nitride III-V compound semiconductor are GaN, AlGaN and GaInN.
The p-type impurity of the p-type nitride compound semiconductor may be selected from at least one Group II element, such as, Mg, Zn and Cd.
The nitride compound semiconductor is typically grown in vapor phase by metal organic chemical vapor deposition (MOCVD).
The method for growing a nitride compound semiconductor having the above composition according to the invention, by using a nitrogen source material that does not releasing hydrogen during release of nitrogen, prevents hydrogen from being caught in the grown crystal and inactivating the dopants or impurities or carriers. Therefore, a p-type nitride compound semiconductor with a high carrier concentration can be obtained without annealing being required for activation of an impurity after growth of the nitride compound semiconductor.